Download In the blink of an eye In the blink of an eye

21/09/07 OPTOMETRY TODAY
CLINICAL ADVICE
In the blink
of an eye
By Karen French and Jane Veys
About the authors
Karen French PhD BSc (Hons)
MCOptom is a practicing
optometrist in
Cambridgeshire. With a PhD in
contact lens material
properties, she has authored
several continuing education
articles in the area of contact
lenses, and also teaches at
City University. Jane Veys
MSc MCOptom FBCLA FAAO
is Education Director for The
Vision Care Institute™ ,
Johnson & Johnson Vision
Care Companies, Europe,
Middle East & Africa. She has
authored an award winning
online continuing education
series and a textbook on
Contact Lens Practice.
D
id you know that in a lifetime
we will spend the same
amount of time blinking as
we do eating? We spend
about five years with our eyes shut
because we are blinking. The average
person blinks 15 to 20 times a minute,
which amounts to more than six
million blinks a year.
So why do we blink?
Blinking is a rapid closure movement
of the eyelids lasting 300-400ms.
Reflex blinking occurs in response to
external stimuli and protects the eye
from foreign particles, bright light and
from injury. Spontaneous blinking is
the regular blinking that occurs with
no apparent external stimulus. Its
main purpose is to moisturise the eye
by forming the tear film, to wipe debris
from the surface of the eye and to aid
tear drainage.
Blinking and the tear film
The eyelids are essential for the
distribution and drainage of tears.
Blinking helps to spread lacrimal fluid
down and across the eyeball. The
downward action of the upper lid
wipes away debris from the anterior
surface of the eye into the lower tear
meniscus. The retraction of the upper
lid then pulls tear fluid from the lower
tear meniscus up over the anterior
surface of the eye, restoring the preocular tear film1. The goblet cells
within the tarsal conjunctiva produce
mucus which is spread over the
corneal epithelium during a blink to
form the deepest layer of the tear film.
Efficient blinking maximises the
spread and distribution of the mucin
over the cornea. Bringing the lids
together during blinking promotes
secretion of lipids from the meibomian
glands2. The inner edge of the lid
margin then spreads these lipids over
the tear film with each blink. This
mechanism is illustrated in Figure 1.
Blinking is also important in tear
drainage, which is an active process
mediated by the contraction of the
orbicularis oculi muscle3. Tears
collect at the medial canthal angle
assisted by the medial movement of
the lower lid with each blink. They
are drawn into the superior and
inferior puncta, and then enter the
lacrimal sac via the canaliculi.
From the lacrimal sac tears drain
into the nasolacrimal duct and into
the nasal cavity.
average of 15 blinks per minute to
approximately 8 blinks per minute5.
More demanding visual tasks can
reduce blink rate even further6.
Blink rate is also reduced in down
gaze5. One of the most widely
reported factors affecting blink rate
is the use of the computer. In
normal healthy people there is on
average a five – fold decrease in
blink rate during screen use7. It has
been suggested a mechanism exists
for inhibiting spontaneous blinking,
sustaining the time between blinks
until difficult target recognition
tasks have been completed8.
Blinking can be affected by
emotional states, increasing with
anger, anxiety or excitement and
also with tiredness9.
The
relationship between blinking and
emotional state is connected to
dopamine activity in the brain
where increased dopamine activity
results in increased blink rate. In
fact many researchers in the field of
psychology use spontaneous blink
rate as a non-invasive correlate of
dopamine function10.
Patients
suffering from certain psychoses,
such as schizophrenia, have
increased blink rates, whilst
Parkinson’s disease, characterised
by low levels of dopamine, results
in decreased blink rates11.
Many authors report the fact that
blink rate is affected by ocular
surface condition. As one of the
functions of blinking is to reestablish a stable tear film, it seems
reasonable to assume that thinning
and break-up of the tear film may
act as a trigger for the next blink.
Indeed it has been shown that there
is a significant relationship between
tear break-up time and blink rate12.
Provocative environments that may
affect the tear film will also increase
blink rate, for example air
conditioning, central heating, low
humidity, cigarette smoke or windy
environments.
Newborn babies
have a very low spontaneous blink
rate, which has been associated
with a thick, stable lipid layer
increasing the stability of the tear
film13. Topical anaesthesia of the
corneal surface reduces blink rate
suggesting that corneal sensitivity to
tear break-up or other surface
change is involved in blink
regulation14.
Blinking and contact lenses
Contrary to popular clinical
opinion, much of the literature
suggests that a contact lens has little
effect on spontaneous blink rate6,8.
Because the contact lens forms an
artificial surface over the cornea,
there is a reduced sense of
imminent tear break-up on the
anterior surface of the lens. As a
result blink rate is less affected by
tear film stability on the surface of
the contact lens than on the surface
of the cornea15.
Blink rate in contact lens wear is
mainly affected by comfort levels.
Reflex blink rate may increase
initially before adaptation, as a
result of the increased foreign body
sensation experienced by the lid
margins. Similarly a poorly fitting
lens may also increase reflex
blinking. The modulus of a contact
Factors affecting blinking
Blink rate is usually fairly constant
for any individual but it can be
affected by external conditions4. In
the literature average blink rates are
quoted as anywhere from 10 to 20
blinks per minute5.
Mental activity can have a
significant effect on blink rate.
Conversation and verbal recall can
increase the rate to in excess of 20
blinks per minute. However, certain
types of mental activity can reduce
blink rate. General reading has been
shown to reduce the rate from an
Figure 1: During blinking there is a large downwards movement of the upper lid,
with a small medial movement of the lower lid
(courtesy of Johnson and Johnson Vision Care Companies)
21/09/07 OPTOMETRY TODAY
CLINICAL ADVICE
21/09/07 OPTOMETRY TODAY
CLINICAL ADVICE
lens material relates to its stiffness and
hence its mechanical resistance to
shape change during blinking. A lens
that has a high modulus is less likely
to follow the corneal contour during a
blink. This may lead to edge fluting
and increased awareness of the lens
edge on the upper lid margin with
each lid movement.
The lubricity of a contact lens
material is a measure of how well the
material resists friction. In particular
the term relates to the level of friction
sustained by the eyelid travelling over
the lens surface with each blink,
particularly if the pre-lens tear film is
inadequate.
Lenses with a low
co-efficient of friction ie higher
lubricity, may result in less irritation
to the upper lid during blinking and
give the lens a smooth feel. Increased
friction on the upper lid margin as it
moves across a lens surface with poor
wettability, in particular a heavily
deposited lens, is more likely to
increase blink rate16. Given that the
eye blinks approximately 10,000 times
a day or more, the impact of contact
lens material and consequent lens
comfort is important17.
A high percentage of contact lens
wearers with symptoms of dryness
even in the absence of other clinical
signs such as corneal staining and
reduced tear break-up time have been
shown
to
have
lid
wiper
The ‘lid
epitheliopathy (LWE)18.
wiper’ is the portion of the marginal
conjunctiva of the upper lid that
contacts the ocular surface and
spreads the tears during blinking
(Figure 2). If the tear film provides
inadequate lubrication between the lid
wiper and the ocular surface there will
be increased friction with the ocular
surface during blinking. This can
result in trauma to the epithelium of
the lid wiper and results in a clinically
observable change best seen by
staining with a combination of
fluorescein and rose bengal. When
using fluorescein, observation is made
easier by instilling two drops five
minutes apart. The extent and severity
of staining can be graded.
Incomplete blinking
It has been demonstrated that 10-20%
of people show incomplete blinking
whilst carrying out visual tasks. The
rate of incomplete blinking may vary
widely between individuals and will
also depend on external factors such as
ambient conditions, fatigue, mental
alertness and difficulty of reading task.
Figure 2a: Schematic representation of the ‘lid wiper’
Figure 2b: Staining typically seen in LWE
(courtesy of Don Korb)
McMonnies16 suggests that incomplete
blinking may represent an attempt to
inhibit spontaneous blinking whilst
concentrating on a visually demanding
task.
Hence increased rates of
incomplete blinking may be directly
related to visually or intellectually
challenging activity. The consequence
of incomplete blinking is a thinner tear
Factors that may increase blink rate
• Conversation
• Anxiety
• Tiredness
• Provocative environmental conditions (air
conditioning, central heating, smoky
atmosphere)
• Poor tear film stability
Factors that may decrease blink rate
• Reading
• Increased difficulty of visual task
• Computer use
• Corneal anaesthesia
Table 1: Conditions affecting blink rate
film in the inferior cornea, which is
more prone to instability. The longer
inter-blink
periods
before
reestablishing the tear film in this area
leads to greater tear film evaporation
and subsequent desiccation of the
inferior cornea. Incomplete blinking
also decreases the integrity of the
mucin and lipid layers within the tears
that contributes to the faster tear breakup in the inferior cornea.
McMonnies suggests that the rate of
incomplete blinking may increase in
patients following laser refractive
surgery, possibly as a result of trauma
to
corneal
nerves
during
photoablation. Contact lens wear can
also influence blinking efficiency.
Reduced comfort associated with
poorly fitting or poorly wetting lenses
may cause a decrease in the
completeness of blinking.
Blinking efficiency exercises
The exposure keratopathy and dryness
symptoms that often accompany
reduced or incomplete blinking can be
managed by encouraging complete and
efficient
blinking
habits.
proposes
blinking
McMonnies18
efficiency exercises.
Patients are
instructed to practise 24 complete
blinks in a period of no longer than 30
seconds. It is suggested they carry out
this exercise every half hour for at
least 1 week. The emphasis is placed
on full and complete blinks, with a
relaxed and light appearance that
looks natural, without involving the
muscles of the eyebrows or cheeks.
Blinks should be rapid, taking about a
third of a second to complete.
Obviously patient education is key in
encouraging patients to carry out the
exercises. It is important to make the
patient aware of the significance of the
problem of incomplete blinking and
the advantages to be gained from
carrying out the exercises. This can be
aided by showing photographs such as
that in Figure 3. McMonnies has
developed a take-home leaflet for his
patients, the Blink Instruction Guide,
which contains explanations and
illustrations of why efficient blinking
is important, as well as instructions for
how to carry out blinking efficiency
exercises.
Other management strategies for the
signs and the symptoms of dryness
include recommending artificial tear
substitutes and/or contact lens
rewetting drops. McMonnies suggests
that carrying out blink efficiency
exercises immediately after drop
instillation will allow the upper lid
margin to have the effect of massaging
the lubricant drops onto the ocular
surface and increasing the benefit of
these drops in aiding the healing of
exposure keratopathy.
In contact lens wearers it is
important in the management of
Factors that may contribute to increased
rates of incomplete blinking
• Provocative environmental conditions (air
conditioning, central heating, smoky atmosphere)
• Fatigue
• Mental alertness
• Computer use
• Increased difficulty of visual task
• Refractive surgery
• Contact lenses
Table 2: Conditions influencing incomplete
blinking
dryness and incomplete blinking not
to ignore the obvious possibility of
improving the contact lens material
where appropriate. Solutions may
include switching to more frequent
replacement lenses to ensure the lens
maintains minimal levels of surface
deposits, using a material with
enhanced wettability and lubricity,
and ensuring an efficient cleaning
regime.
Summary
The awareness of the need for regular
and complete blinking to maintain the
health and comfort of the eye should
not be underestimated.
In the
consulting room complaints of ocular
discomfort associated with dryness
Figure 3: Fluorescein staining highlighting
the exposure keratopathy that can result
from incomplete blinking
(courtesy of Trusit Dave)
amongst both contact lens wearers and
non contact lens wearers are all too
common. The blink mechanism plays
an important role in moisturising the
eye and any reduction in blink rate or
blink efficiency is likely to result in an
increased rate of reported symptoms.
People at greatest risk of problems are
those involved in low blink rate
activities such as reading and
computer use. Where incomplete
blinking appears to be implicated as
the cause of problems, blinking
efficiency exercises may offer a
prospect for relief from ocular
discomfort. The therapeutic effect of
tear lubricants and blinking efficiency
exercises may be enhanced when
combined.
Acknowledgements
Thank you to Dr Kathy Osborn for her
expert review.
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An educational initiative supported by
Johnson & Johnson Vision Care
Institute and Johnson Medical Limited.
21/09/07 OPTOMETRY TODAY
CLINICAL ADVICE